This invention relates to a method of producing an optical element which may suitably be used as, for example, an optical component of a semiconductor manufacturing reduction projection exposure apparatus, such as a phase modulation plate or an optical element having a two-dimensional binary structure or a phase type CGH (Computer Generated Hologram), or an optical interconnection element.
FIG. 9A is a fragmentary sectional view of a Fresnel lens 1, which comprises a Fresnel type diffraction grating. It has a saw-tooth blazed shape 2. FIG. 9B is a fragmentary sectional view of a binary optics element 3, which comprises a binary type diffraction grating. It has a step-like shape 4.
An idealistic diffractive optical element may be one having a blazed shape 2 such as shown in FIG. 9A, and it may assure a diffraction grating of 100% with respect to a design wavelength. However, it is very difficult to produce a complete blazed shape 2. Therefore, while quantizing and approximating the blazed shape 2, the binary optics element 3 having a step-like shape 4 such as shown in FIG. 9B is used. Although the binary optics element 3 is made on the basis of approximation of the Fresnel lens 1, a diffraction efficiency for first-order diffraction light can be 90% or more. In consideration of this, much research has been done with respect to such binary optics 3, by approximating a blazed shape 2 with a step-like shape.
In order that such an optical element has an enlarged power and that, through much better approximation, the optical element has an improved performance as a diffractive optical element, the processed linewidth should be made to be very fine, as much as possible. To this end, a lithographic process having been developed in the semiconductor manufacturing technology and being able to accomplish very high precision processing has been used.
FIG. 10 is a schematic view which illustrates the procedure for making a diffractive optical element having an eight-level step structure. More specifically, in (a) of FIG. 10, drops of a resist material are applied to a cleaned substrate 11 and, through spin coating, a resist coating of a film thickness of about 1 micron is formed on the substrate. Then, through a baking process, a resist film 12a is produced. In (b) of FIG. 10, the substrate 11 is loaded into an exposure apparatus having a performance with which a finest diffraction pattern can be printed. Then, a reticle 13a corresponding to a desired diffraction pattern is used as a mask, and exposure light L, with respect to which the resist film 12a has a sensitivity, is projected thereto. When a positive type resist is used, zones having been exposed with the exposure light L become solvable by a developing liquid. Thus, in (c) of FIG. 10, a resist pattern 14a having a desired size can be produced. Subsequently, in (d) of FIG. 10, the substrate 11 is introduced into an ion beam etching apparatus or a reactive ion etching apparatus by which an anisotropic etching process can be done. While using the produced resist pattern 14a as an etching mask, the substrate 11 is etched for a predetermined time period, by which it is etched to a predetermined depth. Then, the resist pattern 14a is removed. By this, a pattern 15a having a two-level step structure such as shown in (e) of FIG. 10 is produced.
Again, as in (a) of FIG. 10, a resist film 12b is produced on the substrate 11 having the pattern 15a formed thereon. In (f) of FIG. 10, the substrate then is placed in the exposure apparatus, and an exposure process is performed while a reticle 13b formed with a pattern having a periodicity two times larger than that of the reticle 13a is used as a mask. The mask pattern is transferred onto the pattern 15a, after the two are aligned with each other at the alignment precision of the exposure apparatus. By a developing process in (g) of FIG. 10, the resist film 12b is developed such that a resist patten 14b is produced. Then, in (h) of FIG. 10, as in (d) of FIG. 10, a dry etching process is performed to remove the resist pattern 14b, by which a pattern 15b having a four-level step structure is produced.
Subsequently, in (i) of FIG. 10, as in (a) of FIG. 10, again a resist film 12c is applied to the substrate 11. Then, an exposure process is performed while a reticle 13c formed with a pattern having a periodicity four times larger than that of the reticle 13a is used as a mask. Then, in (j) of FIG. 10, the resist film 12c is developed, by which a resist pattern 14c is produced. Finally, in (k) of FIG. 10, the resist pattern 14c is removed, whereby a diffractive optical element with a pattern 15c having an eight-level step-like shape is produced.
When a diffractive optical element having a multiple-level step-like shape is to be produced while reticles having periodicities in multiples are used as masks, such as described above, as long as no alignment error or no dimensional error occurs, a multiple-level step structure having an idealistic shape can be produced. For example, by using three reticles 21a-21c shown in FIG. 11, an eight-level structure xe2x80x9cAxe2x80x9d having an idealistic step height xe2x80x9cdxe2x80x9d can be produced.
A paper in xe2x80x9cO plus Exe2x80x9d, No. 11, pages 95-100 (1996), discusses a procedure wherein processes of resist application, mask pattern and etching are repeated, and it mentions that a multiple-level phase type CGH (Computer Generated Hologram) having phase levels of 2L can be produced where L is the number of masks used.
FIGS. 12a-12c are plan views for explaining a procedure for producing a CGH. More specifically, FIGS. 12a-12c show patterns of reticles 31a, 31b and 31c, wherein zones depicted with hatching are light blocking portions. For example, the reticle 31a is used to perform the etching to a depth of 61 nm, the reticle 31b is used to perform the etching to a depth of 122 nm, and the reticle 31c is used to perform the etching to a depth of 244 nm. Although the order of using these reticles 31a-31c is not fixed, a better resist patterning precision is attainable when a reticle for a smaller etching depth is used first.
The reticle 31a is used first to perform resist patterning on a substrate, and it is etched to a depth of 61 nm. As a result, an etching depth distribution such as shown in FIG. 13A is produced, wherein numerals denote etching depths (nm). Thereafter, the resist on the substrate is removed. Then, the reticle 31b is used to perform resist patterning, and the substrate is etched to a depth of 122 nm, by which an etching depth distribution such as shown in FIG. 13B is produced. Further, the resist on the substrate is removed, and the reticle 31c is used to perform resist patterning. The substrate is etched to a depth of 244 nm, by which an etching depth distribution such as shown in FIG. 13C is produced. FIG. 14 is a sectional view taken along a line E-e in FIG. 13C.
In the examples described above, if an alignment error occurs in registration of reticles, it directly leads to degradation of the performance of a diffractive optical element. More specifically, at the boundary of zones where phase differences are quantized, a very small structure which is not included in the design may be produced due to the alignment error. Such a very small structure causes degradation of the function of the diffractive optical element, and thus, causes a decrease of the diffraction efficiency. Further, the light corresponding to the decrease of diffraction efficiency advances in a direction not intended in the design as unwanted diffraction light, and it causes various undesirable problems. In this manner, light rays not desired may be produced from the diffractive optical element. Such light rays may function as flare light in an optical system wherein the diffractive optical element is used, and the flare light causes degradation of the imaging performance of the optical system.
FIG. 15 shows an example. If an alignment error occurs in the registration of reticles 21a-21c of FIG. 15 and it causes deviations r1 and r2 as illustrated, a produced diffractive optical element has a shape xe2x80x9cBxe2x80x9d different from the idealistic shape xe2x80x9cAxe2x80x9d of FIG. 11. Particularly, very small structures being smaller than the wavelength and produced at the deviated portions of the reticles may cause various problems such as increased scattered light or trapping of light which may result in a temperature rise of the optical element, for example.
It is an object of the present invention to provide a method of producing a diffractive optical element by which a required shape can be formed very accurately.
In accordance with an aspect of the present invention, there is provided a method of producing an optical element, including a process for forming a quantized level distribution on a substrate through lithographic technology, characterized in that, in a portion of the substrate about a boundary between unit zones, in each of which a minimum unit of the level distribution is to be defined, a mask having a width including the boundary and being smaller than the width of the unit zone is provided. This enables that, while the portion about the boundary is kept as an unprocessed region, the remaining portion of the substrate is processed, by which the quantized level distribution is produced on the substrate.
In accordance with another aspect of the present invention, there is provided a method of producing an optical element, including a process for forming a quantized level distribution on a substrate through lithographic technology, characterized in that, in a portion of the substrate about a boundary between unit zones, in each of which a minimum unit of the level distribution is to be defined, a mask having a width including the boundary and being smaller than the width of the unit zone is formed. This enables that, while the portion about the boundary is kept as an unprocessed region, the remaining portion of the substrate is etched, by which the quantized level distribution is produced on the substrate, and that, after the level distribution is produced, the unprocessed portion is removed.
In these aspects of the present invention, the quantized level distribution may have a function as a diffraction grating.
The mask may be provided so that a position of an edge of the mask is registered with the boundary.
The mask may be provided by a lithographic process using an electron beam.
The mask may be provided by a lithographic process using a phase shift mask.
The unprocessed region may have a width not less than an alignment precision of a lithographic apparatus to be used for the production.
The mask may comprise a light blocking film having an etching rate smaller than that of the substrate.
The light blocking film may be substantially made of one of Cr, Al, Ti, Ni, Mo and W.
The surface of the light blocking film may have an anti-reflection function with respect to a wavelength to be used.
The light blocking film may comprise a CrO2 film.
The mask may be made of a light transmissive material having an etching rate smaller than that of the substrate.
The light transmissive material may be substantially made of one of TiO2 and indium tin oxide (ITO).
The unprocessed region may be defined only in a portion of boundaries providing different phase differences.
A portion of the unprocessed region may have a central position being registered with the boundary position of the quantized zones.
At a boundary where a phase difference of zones across that boundary corresponds to a unit period of a design wavelength of the optical element, the unprocessed region may be defined to be included in a region of the substrate which is juxtaposed to the unprocessed region and which is not processed, such that the position of the boundary may be registered with an edge position of the unprocessed region.
The unprocessed region may be defined to be included in one of the adjacent zones in which the amount of processing of the substrate is smaller than the other, and the position of the boundary may be registered with the position of an edge of the unprocessed region.
The removal of the unprocessed region may include an isotropic etching process.
The substrate may be made of fluorite, and the isotropic etching process may comprise a wet etching process using water or a nitric acid.
The substrate may be made of quartz, and the isotropic etching process may comprise a wet etching process using water or a hydrofluoric acid.
The substrate may be made of one of quartz and fluorite.
In accordance with a further aspect of the present invention, there is provided an optical element produced in accordance with a procedure as discussed above.
The optical element may be a computer generated hologram.